The invention relates to a transporting device having self-propelled workpiece carriers each constructed as a car, carriage or the like and having a system to control movement of the carriers. Such a transporting device is described in European patent application EP-OS 0 285 527. The device comprises a rail network on which autonomous cars move. The cars carry their own drive and their own power source with them. Stations, at which workpieces transported by the car are machined, are arranged along the rail network. Every car is equipped with a plurality of sensors and switches, by means of which travel control commands are transmitted to the car by means of signal transmitters arranged in the travel path. An optical distance sensor directed to the front is arranged at the front side. An inductive sensor which is actuated by means of signal strips arranged on the drive path is located on the bottom of the car. In addition, a mechanical switch which is actuated by means of barriers arranged in the travel path is located on the bottom of the car. An additional switch constructed in the form of a rocker is located at the front side of the car. This sensor switch system controls all movements of the car in normal operation. A car travels at maximum drive output insofar as there is no other control signal. Signal strips are arranged on the travel path prior to stopping points. The car initiates a braking process which reduces the speed of the car to a minimum residual speed after detecting the signal strip by means of the inductive sensor on the bottom. It reaches the stopping point at this crawl speed where it is stopped by means of the actuation of the switch on the bottom via a barrier arranged in the travel path. After lifting up the barrier, the car resumes movement at maximum output. If the distance sensor detects an obstacle or a car moving ahead of it, the car likewise brakes to crawl speed. If the obstacle is immovable, it runs against the obstacle at crawl speed. The impact then actuates the rocker at the front side and the car is accordingly stopped. If the obstacle is removed and/or the pressure on the front rocker is relieved, the car resumes its movement at full output. To a limited degree, the sensor switch system makes it possible to control the car also in irregular operating situations, e.g. when there is an obstacle on the travel path. This is detected by the distance sensor which then initiates a braking process. The car collides against the obstacle at crawl speed and is stopped by means of the front rocker which is accordingly actuated.
However, if the distance sensor is defective or does not detect the obstacle for other reasons, the impact against the obstacle takes place at full speed. The car is also stopped by means of the actuation of the front rocker in this instance. Movement continues when the pressure force on the front rocker is removed. In the case of movable obstacles, this can result in the car pushing the obstacle backwards in front of it without being decisively stopped. In the same situation, if the distance sensor is operational but the front rocker is defective this results in the car "driving against" the obstacle, wherein the car constantly tries to achieve the crawl speed. With heavy or virtually immovable obstacles this can easily lead to an overloading of the motor. There is an increased risk of personal accidents if e.g. a hand is pinched by a car with a defective distance sensor. On the other hand, if the inductive sensor on the bottom of the car is defective this results in the car driving into a stopping point at maximum speed, where it is abruptly stopped by means of the actuation of the switch on the bottom. After the stop barrier is removed, the car resumes its travel at maximum output and moves into the next stopping point in the same manner, that is, at maximum speed. Thus, the defective bottom inductive sensor is not discovered.
It is the object of the present invention to increase the reliability of such a transporting device.